ocaml-conpool: enable MDX on lib/{config,endpoint}.mli
Both .mli files document log-source configuration with a code block
that was a bare expression. MDX expects let-bindings; convert to
'let () = ...; assert ...' so the build catches drift in the Logs.Src
API. Add the (mdx ...) stanza wiring config.mli and endpoint.mli to
'conpool' and 'logs'.